1. Field of the Invention
The present invention relates to a load control device utilizing a power control element such as a solid-state relay and a bi-directional gate-controlled triode thyristor. The present invention, particularly, relates to a load control device utilizing a snubber circuit connected to the power control element in parallel.
2. Description of the Prior Art
As an example of a conventional load control device using an alternating-current power supply and a power control element, a load control device utilizing a solid-state relay is described hereinafter. Here, the solid-state relay is a semiconductor contactless relay using a power semiconductor device such as a bi-directional gate-controlled triode thyristor and a reverse-blocking triode thyristor, each of which exhibits characteristics in that it remains in an ON state once the device is turned on, without a control signal to turn it on and off being fed, until current flowing through a switching portion thereof becomes zero.
FIG. 10 is a schematic circuit diagram showing a conventional load control device comprising a solid-state relay. In FIG. 10, reference numeral 1 indicates a solid-state relay, reference numeral 2 indicates a snubber circuit, reference numeral 3 indicates a load, all of which are connected to an alternating-current power supply 4 as illustrated. In this case, the load 3 and the solid-state relay 1 are connected in series to the alternating-current power supply 4 that supplies a power supply voltage Vf. On the other hand, the snubber circuit 2 is connected to the solid-state relay 1 in parallel. Here, the snubber circuit 2 is composed of a capacitor 21 and a resistor 22 connected in series.
The solid-state relay 1 shown in FIG. 10 is composed of a light-emitting element 11 (this usually being a gallium arsenide LED or a gallium aluminum arsenide LED) for converting electrical signals to light signals, a light-receiving element 12 (this usually being a bi-directional photo-gate-controlled triode thyristor that is brought into conduction when light hits the gate thereof) for converting light signals to electrical signals, and a power control element 13 (this usually being a bi-directional gate-controlled triode thyristor). Voltage measured across the power control element 13 is shown as Vs. When a control current I flows through the light-emitting element 11 and a current limiting resistor R1 connected thereto in series, then the light-emitting element 11 emits light; the light-receiving element 12 is brought into conduction; a trigger current flows into a gate of the power control element 13; and the power control element 13 is ignited. After these steps, a load current IL flows through the load 3 so that the load 3 functions.
The snubber circuit 2 connected in parallel with the power control element 13 is necessary for the following reasons. An example in which the load 3 is an inductive load to be regulated by phase control is described by referring to FIG. 11. FIG. 11 is a waveform schematic diagram showing functions of the conventional load control device. Shown from the top to the bottom in FIG. 11 are a waveform of the power supply voltage Vf of the alternating-current power supply 4, a waveform of the voltage Vs appearing across the power control element 13, a waveform of the load current IL of the load 3 flowing through the power control element 13, and a waveform of the control current I.
At time ta0 as shown in FIG. 11, the power control element 13 is ignited by a flow of the control current I as described above, which causes the load current IL to flow through the power control element 13. Although the power control element 13 is kept ignited while the load current IL is flowing therethrough, it is unable to remain ignited at time ta1 when the load current IL becomes zero. Because the load 3 is an inductive load, the phase of the load current IL delays from the phase of the power supply voltage Vf. As a result, because the power supply voltage Vf has already risen to voltage Va1 at time ta1, a voltage Vs having a steep rising edge is applied to the power control element 13. When a rate of voltage increase (dv/dt) exceeds a critical OFF voltage rising rate in commutation of the power control element 13, the power control element 13 may experience a commutation failure. In order to prevent this from happening, the snubber circuit 2 is used to moderate the rate of voltage increase. In other words, high-frequency components in the rising voltage should be removed.
The snubber circuit 2 is also required in order to protect the power control element 13 against a surge voltage. To be more specific, for example, if such an element as a transistor which can shut off a load current even when it is flowing by stopping inputting control signals, is used, it is possible, especially when the load is an inductive load, that a surge voltage exceeding a withstanding voltage of the power control element is generated across the power control element at the instant when the load current is shut off, and thereby destroying the element. Therefore, the snubber circuit is necessary also for suppressing such surge voltages, or in other words, for removing high-frequency components from such voltages.
The snubber circuit is also necessary, especially to an inductive load, for preventing the power control element 13 from being unable to remain ignited and turning off. The reasons are described hereinafter with reference to FIG. 12 and FIG. 13. FIG. 12 is a schematic circuit diagram showing a conventional load control device similar to the one shown in FIG. 10. In FIG. 12, such elements as are found also in FIG. 10 are identified with the same reference symbols and descriptions about the circuit are not repeated. A portion different from the load control device shown in FIG. 10 is that a bi-directional gate-controlled triode thyristor is used as a power control element instead of a solid-state relay.
In FIG. 12, reference numeral 121 represents a bi-directional gate-controlled triode thyristor. A load 3 and the bi-directional gate-controlled triode thyristor 121 are connected in series to an alternating-current power supply 4. A snubber circuit 2, on the other hand, is connected to the bi-directional gate-controlled triode thyristor 121 in parallel. Reference numeral 122 represents a controller connected externally for feeding a trigger current to a gate of the bi-directional gate-controlled triode thyristor 121. When the trigger current flows, the bi-directional gate-controlled triode thyristor 121 is ignited and a load current IL starts flowing through the load 3 causing the load 3 to start functioning. The bi-directional gate-controlled triode thyristor exhibits characteristics in that it remains ignited once it is turned on without current flowing through the gate thereof until current flowing through an output portion thereof becomes zero.
FIG. 13 is a waveform schematic diagram showing rising portions of currents flowing through major parts of the load control device shown in FIG. 12 immediately after the bi-directional gate-controlled triode thyristor 121 is ignited and when the load 3 is an inductive load. At time tb0 as shown in FIG. 13, a trigger current (not illustrated) flows into the gate of the bi-directional gate-controlled triode thyristor 121 and ignites it. In this case, a total current It flowing through the bi-directional gate-controlled triode thyristor 121 is a sum of the load current IL and a discharge current Is of the snubber circuit 2. Because the load 3 is an inductive load, the load current IL rises gradually.
Once the total current It flowing through the bi-directional gate-controlled triode thyristor 121 exceeds a latching current, the ignited bi-directional gate-controlled triode thyristor 121 remains ignited without having a flow of the trigger current. Here, assume that no snubber circuit 2 is provided. Then, the total current It flowing through the bi-directional gate-controlled triode thyristor 121 is made up of only the load current IL. If the gate current is not present until time tb1 as shown in FIG. 13, the bi-directional gate-controlled triode thyristor 121 is unable to remain ignited and turned off (hereinafter, this phenomenon is called ignition failure), because a value of the load current IL does not exceeds a value of the latching current. In an actual case, there is provided the snubber circuit 2 and the total current It which is made up of the load current IL and the discharge current Is. As shown in FIG. 13, because the total current It has exceeded the latching current since the ignition timing (time tb0), the ignition failure never happens even if the gate current is stopped immediately after the ignition.
In recent years, with advancement in areas of energy savings, miniaturization, and high-performance of all equipments, there has been an increasing trend in number of lighter loads requiring a smaller load current. Accordingly, demands for achieving stable control of these lighter loads have been also increasing.
As mentioned above, If the snubber circuit 2 is connected to the power control element 12 in parallel as shown in FIG. 10 in order to remove the high-frequency components or prevent the ignition failure from occurring, when the power control element 13 is in OFF state, although the amount is small, current flows through a series circuit which is made up of the load 3 and the snubber circuit 2 and connected to the alternating-current power supply 4. This means that current flows through the load 3. Therefore, in conventional technologies, there has been a drawback in which a lighter load that may operate even with such a small current cannot be used as a load.
With regard to the conventional load control device shown in FIG. 10, as an example, given that the voltage of the alternating-current power supply 4 is 200V (rms), resistance of the resistor 22 and capacitance of the capacitor 21 are 22 ohms and 0.022 μF respectively as a snubber circuit constant, the current flowing through the load 3 via the snubber circuit 2 is in the range between 1 mA and 2 mA when the power control element 13 is off. With this amount of current, the load 3, if it is a light load, may fail to work properly. It is also possible that, although the load 3 may not be influenced with this amount of current, the load 3 may pose an unstable state or the like in which the load 3 malfunctions in an instant when the current increases even slightly due to fluctuations of the power supply voltage or the like.
In an attempt to cope with the lighter load, if the current flowing through the snubber circuit 2 is made smaller by increasing impedance thereof so that the lighter load can not malfunction, it is also possible that the aforementioned effects of the snubber circuit 2 become no longer available and a proper control of the load may not be achieved.
Furthermore, Japanese Patent Application Laid-Open No. H3-284121 discloses a switching circuit protective device that shows a method for preventing a load from malfunctioning by connecting a resistor in parallel to the load that is under control of a solid-state relay type switch having a snubber circuit in parallel. The resistor is intended to reduce a voltage generated across the load by current flowing through the snubber circuit and thereby prevent a malfunction of the load caused by that voltage. According to this method, however, a drawback is that the resistor allows unnecessary current to flow therethrough and consumes unnecessary power. Another drawback is that heat being generated by the resistor should be dealt with in designing such a device.